Normas Iso

INTERNATIONAL IS0

STANDARD 5725-l

First edition 1994-I 2-15

Accuracy (trueness and precision) of

measurement methods and results -

Part 1:

General principles and definitions

Exactitude (justesse et fidblit6) des r&ultats et mgthodes de mesure -Partie 1: Principes g&-Graux et d6finitions

Reference number

IS0 5725-l :I 994(E)

Contents

Page 1 Scope .............................................................................................. 1 2 Normative references ..................................................................... 1 3 Definitions ................................................................................. 2 4 Practical implications of the definitions for accuracy experiments 4

41. Standard measurement method ............................................ 4

42. Accuracy experiment .............................................................. 4

4.3 Identical test items ................................................................. 5

44. Short intervals of time ............................................................ 5

45. Participating laboratories ........................................................ 5

4.6 Observation conditions ........................................................... 5

5 Statistical model ........................................................................ 6

51 Basic model ............................................................................ 6

l

52. Relationship between the basic model and the precision ..... 7

53. Alternative models .................................................................. 7 6 Experimental design considerations when estimating accuracy 7

61. Planning of an accuracy experiment ...................................... 7

62. Standard measurement method ............................................ 8

63. Selection of laboratories for the accuracy experiment .......... 8

6.4 Selection of materials to be used for an accuracy experiment 10 7 Utilization of accuracy data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

7.1 Publication of trueness and precision values . . . . . . . . . . . . . . . . . . . . . . . 11

7.2 Practical applications of trueness and precision values . . . . . . . 12

Annexes A Symbols and abbreviations used in IS0 5725 . . . . . . . . . . . . . . . . . . . . . . . 13

0 IS0 1994 All rights reserved. Unless otherwise specified, no part of this publication may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying and microfilm, without permission in writing from the publisher.

International Organization for Standardization

Case Postale 56 CH-1211 Geneve 20 Switzerland

l l

Printed in Switzerland

ii

B Charts of uncertainties for precision measures ..................... 15

C Bibliography ............................................................................ 17

Foreword

IS0 (the international Organization for Standardization) is a worldwide federation of national standards bodies (IS0 member bodies). The work of preparing International Standards is normally carried out through IS0 technical committees. Each member body interested in a subject for which a technical committee has been established has the right to be represented on that committee. International organizations, governmental and non-governmental, in liaison with ISO, also take part in the work. IS0 collaborates closely with the International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.

Draft International Standards adopted by the technical committees are circulated to the member bodies for voting. Publication as an International Standard requires approval by at least 75 % of the member bodies casting a vote.

International Standard IS0 5725-l was prepared by Technical Committee lSO/TC 69, Applications of statistical methods, Subcommittee SC 6, Measurement methods and results.

IS0 5725 consists of the following parts, under the general title Accuracy

(trueness and precision) of measurement methods and results:

- Part 1: General principles and definitions

- Part 2: Basic method for the determination of repeatability and re-producibility of a standard measurement method

- Part 3: Intermediate measures of the precision of a standard measurement method

- Part 4: Basic methods for the determination of the trueness of a standard measurement method

- Part 5: Alternative methods for the determination of the precision of a standard measurement method

- Part 6: Use in practice of accuracy values

Parts 1 to 6 of IS0 5725 together cancel and replace IS0 5725:1986, which has been extended to cover trueness (in addition to precision) and intermediate precision conditions (in addition to repeatability and repro-ducibility conditions).

Annexes A and B form an integral part of this part of IS0 5725. Annex C is for information only.

Introduction

0.1 IS0 5725 uses two terms “trueness” and “precision” to describe the accuracy of a measurement method. “Trueness” refers to the close-ness of agreement between the arithmetic mean of a large number of test results and the true or accepted reference value. “Precision” refers to the closeness of agreement between test results.

0.2 The need to consider “precision” arises because tests performed on presumably identical materials in presumably identical circumstances do not, in general, yield identical results. This is attributed to unavoidable random errors inherent in every measurement procedure; the factors that influence the outcome of a measurement cannot all be completely controlled. In the practical interpretation of measurement data, this vari-ability has to be taken into account. For instance, the difference between a test result and some specified value may be within the scope of un-avoidable random errors, in which case a real deviation from such a specified value has not been established. Similarly, comparing test results from two batches of material will not indicate a fundamental quality dif-ference if the difference between them can be attributed to the inherent variation in the measurement procedure.

0.3 Many different factors (apart from variations between supposedly identical specimens) may contribute to the variability of results from a measurement method, including:

a) the operator;

b) the equipment used;

c) the calibration of the equipment;

d the environment (temperature, humidity, air pollution, etc.);

e) the time elapsed between measurements.

The variability between measurements performed by different operators and/or with different equipment will usually be greater than the variability between measurements carried out within a short interval of time by a single operator using the same equipment.

0.4 The general term for variability between repeated measurements is precision. Two conditions of precision, termed repeatability and reproduc- ibility conditions, have been found necessary and, for many practical cases, useful for describing the variability of a measurement method. Un-der repeatability conditions, factors a) to e) listed above are considered constants and do no contribute to the variability, while under reproduc-ibility conditions the\ vary and do contribute to the variability of the test results. Thus repeatability and reproducibility are the two extremes of precision, the first describing the minimum and the second the maximum variability in results. Other intermediate conditions between these two extreme conditions of precision are also conceivable, when one or more of factors a) to e) are allowed to vary, and are used in certain specified circumstances. Precision is normally expressed in terms of standard devi-ations.

0.5 The “trueness” of a measurement method is of interest when it is possible to conceive of a true value for the property being measured. Al-though, for some measurement methods, the true value cannot be known exactly, it may be possible to have an accepted reference value for the property being measured; for example, if suitable reference materials are available, or if the accepted reference value can be established by refer- ence to another measurement method or by preparation of a known sample. The trueness of the measurement method can be investigated by comparing the accepted reference value with the level of the results given by the measurement method. Trueness is normally expressed in terms of bias. Bias can arise, for example, in chemical analysis if the measurement method fails to extract all of an element, or if the presence of one element interferes with the determination of another.

0.6 The general term accuracy is used in IS0 5725 to refer to both trueness and precision.

The term accuracy was at one time used to cover only the one component now named trueness, but it became clear that to many persons it should imply the total displacement of a result from a reference value, due to random as well as systematic effects.

The term bias has been in use for statistical

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